The present invention relates to a method of dewaxing oils of hydrocarbons with a solvent and in particular a method permitting the dehydration at least in part of the recovered solvent. According to an embodiment, the method allows the adjustment of the solvent/antisolvent ratio and in particular the toluene/methylene/ketone (MEX) ratio.
The bases of distillates for the production of lubricating oils of hydrocarbons recovered during the distillation of petroleum under atmospheric pressure and under vacuum require various treatments adapted to make them comply with the specifications. One of these treatments is the paraffin extraction or dewaxing the purpose of which is to remove the hydrocarbons with a relatively high melting point in particular the paraffins with a linear chain which prevent the lubricating oils from remaining fluid enough at low temperatures. This treatment usually follows a refining making for example use of furfurol as a solvent.
The dewaxing is a well known operation which makes use generally at low temperature of a solvent or a mixture of such solvents that they are dissolving the hydrocarbons with a low melting point whereas the hydrocarbons with a high melting point are precipitating (paraffins) and may be retained on a filter.
Most frequently is used a mixture of solvents, for example a mixture of a good solvent for paraffin such as benzene or toluene hereinafter referred to as "solvent" and of a bad solvent for paraffin such as a ketone having from 3 to 6 carbon atoms, for example the methylethylketone (MEX) of the methylisobutylketone (MIBK) hereinafter referred to as "antisolvent". Here is preferred a mixture of toluene and of (MEX). The mixture of solvent and antisolvent will globally be referred to as "solvent mixture".
For a detailed description of the dewaxing operation, one could refer to the literature and to the patents and for example to the work of P. Wuithier, "Le Petrole, Raffinage et Genie Chimique", (1965), pages 641-649 and to the U.S. Pat. Nos. 4,088,565 and 4,354,921.
One generally operates at low temperature, for example towards -20.degree. to -30.degree. C. and the filtration is usually carried out on rotary vacuum filters, which filter should be periodically cleaned with a solvent or a dry solvent mixture. The recovered solvent mixture is then separated from the oil in any suitable manner, for example by distillation.
Thus the separation of the oils and of the paraffins is obtained by a mechanical effect of filtration of low temperature combined with a chemical effort of dissolution in a solvent mixture the optimum composition of which varies in accordance with the batch.
This unit exhibits the other particular feature of having to treat different batch types originating from different cuts of distillates from a vacuum column, in an alternating manner and this throughout the whole year.
The required amounts of solvent mixture are very great since they vary between 200 and 400% by volume of the batch to be treated.
The solvent mixture is therefore recovered after separation of the oils and of the paraffins in different recovering sections with heat supply and then recycled to the filtration.
The solvent mixture is of a polar nature in view of the presence of the antisolvent which has a hydrophilous nature and has a tendency to fix the water it meets with. This water originates in particular from the batch and from the stripping steam used for recovering the last traces of the solvent mixture in the products.
Taking into account the polar nature of the solvent mixture, its direct complete drying is presently not practiced industrially. It therefore remains a problem which has not been solved in the industry in an economically satisfactory manner which is the one consisting in a fine separation between water and solvent mixture. Another problem which has not been solved in a satisfactory manner is the following:
For each batch type of a dewaxing unit, there are corresponding antisolvent/solvent, for example (MEK) toluene proportions. The dewaxing unit undergoes a frequency batch change (up to twice or three times a week for example) and the operator requires some time before obtaining the good composition of the mixture of solvent and of antisolvent in the whole unit for the next batch, while taking in particular into account the substantial residence time of this unit.
This exhibits the inconvenience of unbalancing the unit, of not letting same operating in an optimum way and of momentarily impairing the oil output yields.
By way of example, the treatment of a "bright stock" is currently using a MEK/toluene weight ratio of 0.7:1 to 1:1 whereas the treatment of light oils currently uses ratios of 1.1:1 to 2.3:1.
It is therefore important to be able to readily produce distinct supplies of solvent mixtures with the different compositions in order to be able to make available through simple mixing of streams issuing from these supplies, a whole range of compositions adapted to the variations of the treated batches.
This problem is referred to the U.S. Pat. No. 4,898,674 but the proposed solution (use of pervaporation membranes) remains industrially applicable with difficulties.